1. Field of the Invention
The present invention generally relates to a multiple stage pump and, more particularly, to a variable displacement multiple stage pump for a hydraulic system.
2. Background Description
Hydraulic pumps are widely used in a vast array of automotive and heavy machinery applications. These applications may include, for example, drive vehicles, powerful hydraulic cylinders and injection systems. In current systems, pump displacement of the hydraulic pump is not adjusted to the needed amount of energy for a desired application. That is, the pump displacement is kept constant. This is mainly due to cost constraints associated with manufacturing and designing variable pump displacement systems. Thus, variable pump systems are not currently or widely used in the automotive industry due to these cost constraints.
However, it is known that fuel economy and other efficiencies can be realized by using variable pump systems. In known variable pump systems, as shown in FIG. 1, on/off switching valves 10 (e.g., 3 way/3 position valve) are located in a common rail line 12 for all of the pumps 14. The on/off switching valve 10, shown in an exploded view of FIG. 1a, uses two pumps to provide three different volumes; namely, (i) a small pump V1 for a small flow, (ii) a large pump V2 for a larger flow and (iii) both pumps together V1 and V2 to have a maximum flow. Thus three different volumes are generated when V1<V2 (e.g., 5 l/m, 10 l/m and 15 l/m). This arrangement, though, creates pressure peaks in the rail line 12 as well as in the pump 14, itself. Also, by using the on/off switching valves 10 in the common rail line 12, both sides (pump and rail sides) will have difficulty with the pressure peaks. That is, the 3 way/3 position valve is a “digital” volume shift which has very little influence to reduce peek pressures during switching. Thus, the pump side must handle the additional load and will have a problem with the resultant durability. Also, with these systems, on the rail side, the pressure peaks change the rail dynamic which, in turn, causes injection variations. The additional volume peak must be handled by the rail pressure regulator valve.
The present invention is directed to overcoming one or more of these problems.